The invention concerns a novel honeycomb monolith structure, especially for use in mass transfer-limited processes. Honeycomb monolith structures comprise a plurality of cell walls defining a plurality of channels, or elongated cells, separated from each other by said cell walls, wherein the plurality of cell walls and channels extend in parallel along a common direction from an entrance end to the outlet end of the structure in the fluid flow direction. Monolith structures are usually given a complementary shape and placed side by side in a reactor, with their channels aligned along the flow direction in a reactor, completely covering the cross-sectional area of the reactor, with the consequence that the gas flowing through the reactor is made to pass entirely through the channels of the monolith structures.
Honeycomb monoliths are, for instance, utilised as structured chemical reactors, either by producing the whole monolith structure in a catalytic material, or by coating the surface of a monolith structure with a catalytically active material wherein the internal monolith structure walls contain an (inert) carrier material. Such monolith reactors are produced in a range of materials, typically different types of metals, ceramics or composites, wherein several production methods are known in the art. Common examples of production routes are extrusion and moulding.
A honeycomb monolith catalyst may be employed to induce and/or increase the rate of several types of chemical reactions such as synthesis of organic and inorganic compounds, decomposition of oxides, oxidation of compounds etc. Such monolith reactors can be produced with a large span in pitches and wall thickness, depending on demands on surface area, conversion, pressure drop, plugging resistance etc., as well as considerations involving monolith material strength and production limitations.
Among the advantages of monolith reactors are a low pressure drop, a relative high surface area, reasonable production costs, and the fact that they can be utilised in processes with gas mixtures containing particular material (dust, fly ash, soot etc.), such as effluent gases from incinerators.
The current invention concerns a novel honeycomb monolith structure having a novel honeycomb monolith channel design that can be used advantageously in monolith reactors for conducting several physicochemical processes, especially for processes with relative fast reactions where the rate-limiting step in the conversion is the mass-transfer to the surface by the reacting components. Mass transfer-limited processes may be catalytic, but may also be non-catalytic such as adsorbents, absorbents, and poison traps. Catalytic processes include any heterogeneous reaction that is mass transfer-limited, including, but not limited to, SCR, nitrous oxide decomposition, ammonia oxidation, natural gas processing, and water-gas shift reaction.
One example of the use of a honeycomb monolith catalyst is its use in NOx-removal from exhaust/flue gases wherein the flue gas often contains particular matter with varying particle size. Nitrogen oxides may be catalytically reduced to elementary nitrogen and water by the use of specific types of ceramic or metallic catalysts (called selective catalytic reduction, SCR). These catalysts can be extruded into a monolith structure. For the NOx removal reactions, the mass-transfer to the monolith surface is the rate-limiting step.
Common SCR catalysts are manufactured from various ceramic materials used as a carrier, such as titanium oxide, and the active catalytic components are usually either oxides of base metals, such as vanadium and tungsten, zeolites and various precious metals. Each catalyst component has advantages and disadvantages. Titanium oxide-based ceramic honeycomb SCR catalysts are often used for power generation, petro-chemical and industrial processing industries.
Honeycomb monolith structures are available wherein the transversal cross section of the channels has different shapes. Such a transversal cross section is often referred to as a cell. The most common commercially available monolith structures are honeycombs with channels having a square transverse cross section, as for example shown in International patent application WO 2012/135387 A1 (Cormetech, Inc., 2012). Also, catalytic converters with channels having a rectangular transverse cross section are known. Such a rectangular shape is, for example, disclosed in U.S. Pat. No. 5,866,080 A (Day, 1999) disclosing a rectangular transverse cross section with a width/height ratio of at least 1.2, preferably in the range of 1.5 to 2.5, and in U.S. Pat. No. 6,258,436 B1 (Siemens AG, 2001), disclosing a rectangular transverse cross section with a width:height ratio of 2:1.
Structures with hexagonal cells are also known. Chinese utility model CN201815314 relates to a honeycomb catalyst, provided with a regular hexagonal internal pore passage structure and used for SCR denitration technology. The regular hexagonal internal pore passage combines the advantages of a square internal pore passage and a circular internal pore passage. The plurality of flue gas flow internal pore passages distributed in honeycombed shapes are arranged in a square or hexagonal catalyst skeleton, and the transverse cross section of each internal pore passage is regular hexagonal, having a width:height ratio of about 1:1.
A disadvantage with the channels in prior art monolith structures is the high density of corners (corners per cm2) and/or the fact that a majority of the corners are straight corners, i.e. corners wherein two adjacent walls meet at an angle of 90 degree. Examples are the ubiquitous square channel/cell geometry, as well as the hexagonal shaped channels/cells.
One of the challenges with the prior art is that the corners, especially sharp corners (e.g. corners of 90 degrees or less), have undesirable properties, such as a low chemical conversion, a higher pressure drop and are prone to plugging and fouling with particulate material in the gas stream, with subsequent and accompanying erosion problems.
Published patent documents also exist on smoothing walls and corners in monolith structures in order to obtain a structure with an increased structural strength, as is, for example, described in US patent application 2010/0062213 A1 (Denso Corporation, 2010), which discloses an hexagonal honeycomb structure with slightly curved walls and smoothed angles between two adjacent walls, and in U.S. Pat. No. 5,714,228 (General Motors Corporation, 1999) which discloses a hexagonal shape with rounded corners.
A monolith structure with only circular channels will have the main disadvantage that there is no “close packing” in the plane, which gives a much lower open frontal area (OFA) with circular channels compared to e.g. square channels. Low open frontal area leads, among other things, to an undesired high pressure drop and more frequent impact by fluidized particles in the incoming gas flow.
Monolith structures are also used in filters. In WO2010/149908 (Saint-Gobain, 2010) it is described a monolith structure for gas filtration comprising convex polygon shaped channels. The channels being alternately plugged at one or the other of the ends of the structure so as to define inlet channels and outlet channels for the gas to be filtered. The inlet and outlet channels have different shapes within the same structure. The walls may further comprise a catalyst. Such filter systems are also described in WO2005/115589 (Robert Bosch, 2005) and in US2007/0227109 (Ibiden, 2007). In such structures there will be different flow characteristic and different residence time in the different channels because of the different shapes. In the filters as described, a significant contribution to pressure drop will be the filter area formed by the cell walls between the inlet and outlet channels as the fluid has to travel through the walls. The aim of our invention is to limit the pressure drop in the fluid flow into and along the parallel channels combined with large surface areas and low risk of dust accumulation. This problem is not solved by the filters as described in the prior art.
In U.S. Pat. No. 3,502,596 (Sowards, 1965) it is described the use of a honeycomb structure as packing material in chemical process vessels and as catalyst supports in chemical reactors. The honeycombs themselves are regular LL/LS=1, but each element is cut as a cube or prism.